Special Agronomist, (Winter Crops), Department of Agriculture, Dubbo.
Farmers have been using herbicides in tillage systems since the early 1950’s. Skeleton weed was a problem attacked by numerous methods including tillage. With the advent of 2,4-D a system of control was developed, including herbicides. Significantly 2, 4-D did not directly lead to measured yield increases, but was and still is primarily used as an aid to easier harvesting.
One aim of tillage is weed control and herbicides offer a possible alternative. However, they provide little assistance in breaking down trash, aerating soils, controlling erosion, breaking hard pans and levelling the soil surface. These, together with “making a seed bed”, “improving the tilth”, controlling other pests are some of the claims for tillage.
New South Wales cereal producers can increase yields above accepted levels by using, in addition to currently accepted practices, more fertilizer, timely sowing and controlling weeds effectively, Herbicides are useful in assisting with sowing on time and controlling weeds.
Despite the desires of many, herbicides are only “tools” within the overall system of tillage and crop production. They are far from “magic wands” and in most cases often demand a high level of management and some compromise. Therefore, there are merits in developing crop establishment systems which do not demand herbicides.
Developments have raced ahead during this period. However, we have seen almost no advances in late post-emergent herbicides, mainly due to the difficulty of being able to demonstrate profitable yield increases. Postulation as to what developments will occur in the next 20 years and the systems which will be developed in association are exciting.
The 1960’s saw a range of early post-emergent herbicides developed and successfully employed which control a wide range of broadleaf weeds. These included bromoxynil, diuron, linuron and others which have dropped by the wayside for various reasons, but primarily due to crop damage, lack of weed control or cost. Also, the first of the "grass killers” came onto the market as a major breakthrough. By current day standards some were rather crude, yet others have stood the test of time and are widely used today - Avadex BW (R) and Neoban (R). Atrazine came onto the scene for use in sorghum at this time. Today atrazine is being closely looked at as a residual herbicide for controlling weeds on f allows for 6 to 8 months in areas where fallowing pays,
Also during this period the bipyridyls came on to the scene and were initially used in pastures. Around 1966 a few brave souls attempted to replace tillage with herbicides for crop production with the likes of Gramoxone (R) and Reglone (R), either in combination or alone, depending upon weeds. They plugged on until the next major breakthrough with the release of Hoegrass (R) in 1977. This allowed in crop control of the major boggey-ryegrass.
The early 1970’s saw an explosion of the use of soil incorporated or pre-emergent herbicides. Trifluralin, Stomp (R), Avadex BW (R) and mixtures of these were seen initially as “the answer to the grass weed problem”. Never let us forget the damage done to soils by the adoption of these methods,
In addition to the bipyridyls, Roundup (R) came on the scene in the early 1970’s and it too was initially looked upon for pasture weed control. However, time has shown it to be a very useful tool for controlling both perennial as well as annual weeds for cropping, Spray seed (R) started the chemical ploughing revolution and Roundup (R) has built upon the excellent early groundwork,
Knockdown herbicides currently being used have no selectivity to any emerged crop and nor do they provide any residual control. A more ideal product would combine extended residual weed control with good crop selectivity as well as knockdown capability.
The next step saw the use of tank mixes of both knockdown and in crop residual herbicides, e.g. Roundup (R) and Yield (R). In practice, however, results have been variable and more knowledge on the effect of stubble, green plant material, soil moisture and the period between application and incorporation is required.
Most weeds can be satisfactorily controlled where a planned approach is taken to each individual paddock. In a direct drill situation a knockdown herbicide just before the sowing operation is not always needed and should not always be advocated. Combines with all tines working at sowing can do an effective job of initial weed control in some situations.
Users of herbicides will continually, with intention or accident, use higher or lower herbicide rates than those suggested on labels. Producers who are substituting tillage with herbicides in the f allowing situation have quickly learnt of problems of application. Reduced water volumes are the only way to apply knockdown herbicides over a large area many times. In addition, the use of herbicide carriers other than water offer advantages in some situations, Accurate application employing “tramlines” as marking aids can avoid missing areas, Weed seed build-up can increase from areas missed.
With a succession of applications of knockdowns, the chances of weeds not being controlled are increased. Such systems are not forgiving of management inadequacies,
In the northern wheat belt of New South Wales no till f allows are replacing all cultivations with herbicides like Roundup (R), Gramoxone (R), 2,4-D, atrazine and dicamba. All sprayings are knockdowns, except atrazine. Most land is continuously cropped without the involvement of grazing animals so far. Soil erosion by water is the major reason for interest. Therefore maintenance of a good stubble cover is a primary consideration. Total spray frequency has been similar to number of cultivations, with variation over the season. Comparable wheat yields have been measured, and significant yield increases have been harvested from No Till summer crops.
Already we have seen changes in weed types and densities in paddocks where tillage has been replaced with herbicides over a number of seasons. Deep seeded weeds like wild oats have failed to emerge in density, shallow seeded grass weeds like Rat’s tail fescue, barley and brome grasses have shown up to be a problem.
In addition, some tillage systems make it difficult to use some herbicides, specifically soil incorporated herbicides in a stubble mulch system, because of the amount of stubble residue.
There is need for the use of soil residue herbicides to take the pressure off knockdowns, especially in f allowing situations.
The latest development has seen the release of Glean this year. This is the first of no doubt many “multi-purpose” selective crop herbicides. There are wide possibilities of profitable incorporation of such products into cropping programmes. The term "multi-purpose" is used to indicate that such materials could be used for fallow, pre-sowing, post-sowing, or in crop weed control. The herbicide might be applied 3 or 4 times during a l2’~month period. As an example, in traditional fallowing areas a multi.-purpose “ herbicide might well be applied in spring on fallow, then in early autumn following weed germinating rains well before sowing and then again as a top-up early post- emergent, Again, in a stubble crop situation, herbicide could be applied immediately after harvest following rain, and then again early post-emergent, Such herbicides are selective for the cereal and control a wide range of broadleaf and some grass weeds. Of course tank mixes are possible with knockdowns, saving on application costs,
The next step in the stubble crop situation, of course, is to apply the residual herbicide either before harvest or with the harvesting operation. Ideally stubbles will remain untouched, with the development of suitable equipment for sowing into standing stubbles. Where weeds are controlled from harvest to sowing with residual herbicides with savings in cultivation there will be little available grazing.
Rightly, many people are continually concerned as to the likely effect the wide use of chemicals may have on the populations and activity of soil microbes, earth worms, soil insects and other beneficial life in and on our soils, This potted history bears testament to the long period of time some of our common herbicides have been used. This is not to suggest that no damage has occurred over this period, but it does suggest that damage is very minor and very difficult to detect with available monitoring equipment. Research on soil residues with herbicides has been conducted on one site in the UK since 1963 without any measurable changes in “soil life” soil fertility, or quality of product produced. Everyone is encouraged to continue their constant vigil on the effects of chemical usage on the environment.
1. Fryer, J.D. - Herbicides: Do they affect soil fertility? Span 24, 1, 1981.
2. Gammie, R.L. and Dellow, J.J. - Weed Control in Winter Crops -1982.
3. No-Tillage Crop Production in Northern New South Wales -Project Team Meeting 1982 Report.